Reproduction and Development

Growth, Maturation, and Reproduction

Giant pandas are born tiny, helpless creatures. Weighing only about 104 g at birth (Schaller, 1985), the panda cub is small, pink and completely altricial. Its eyes will not open for many weeks, but it can cry loudly for its mother's comfort from birth. Milk teeth begin to appear at 2.5 months, and adult dentition is evident at 14 months (Morris and Morris, 1966).

Pandas grow rapidly to their adult size. Typically, a panda has reached its adult weight by its fourth year. When comparing the relative size of male and female pandas, only a slight sexual dimorphism is evident. Schaller (1985) reports that males of some species of bear may be 29-41% larger than females of the same species. In pandas, however, males are about 18% larger than females. This number can be misleading in that there appears to be great individual variation in adult size, at least in captive populations. For examples, in the fall of 1998, the San Diego Zoo’s breeding pair of pandas weighed approximately 115 kg and 100 kg each, with the female being the heavier of the two.

Pandas become sexually mature between 66-78 months of age (Schaller, 1985). It is speculated that females may mature faster than males, as many captive females appear more receptive to mating before males of the same age appear amenable to courtship. It is unclear at this point whether this is due to a physiological difference in maturation rate, or differences in the rate at which each sex learns appropriate sexual behavior. Even though females appear to mature sooner, young females do show changes in their estrus patterns in the first few years of maturity. This indicates that perhaps females begin showing estrus-like behavior before they are fully mature.

It is estimated that pandas in captivity may live as many as 30 years. In the spring of 2000, the San Diego Zoo male is estimated to be 22-24 years of age, while the National Zoo male was estimated at 28 years of age upon his death in 1999 (FONZ Website, 1999). Pandas in the wild are thought to have much shorter life expectancies.

Giant pandas are monoestrus, generally becoming receptive in the early to late spring. Like other bear species, pandas have a delayed implantation period suspected to range from three to six months. In delayed implantation, embryonic development is arrested in the blastocyst stage (Mead, 1989). Generally, the blastocyst remains the same size until a few days before implantation. At this time, a number of metabolic and cytological changes occur as development begins in the blastocyst. A number of factors may influence the onset of implantation, such as changes in photoperiod, or changes in the uterus, ovaries, pineal or pituitary glands. At this point, it is unclear which factors are most important in determining the onset of implantation in pandas.

Schaller (1985) reports a gestation period of 135 days, although there is considerable variation in gestation periods among captives. For example, a captive female in Wolong, Dong Dong, has given birth after a gestation of 180 days. This lengthy gestation is considered extreme. It is noteworthy that there appears to be considerable variation in gestation periods in females who have endured several pregnancies: Dong Dong, many times a successful mother, has also had gestation periods much closer to those reported by Schaller.

Birth typically occurs in the late summer or fall. Twinning is common in pandas, although it is rare that both offspring of a pregnancy will survive without intervention. The birth of one cub is frequently reported, but triplets are considered rare. Little is known about the infant-rearing techniques of wild panda mothers, although it is common for infants to remain with their mother for up to two years after birth. In captivity, mother-rearing is one of the most troubling aspects of panda conservation, as high infant mortality continues to be a significant obstacle to conservation programs.

Bibliography:

Cockrum, E. (1962) Introduction to Mammalogy. Ronald Press, New York

FONZ (Friends of the National Zoo) Website (1999). http://www.fonz.org

Mead, R. (1989) The physiology and evolution of delayed implantation in Carnivores. In Carnivore Behavior, Ecology and Evolution (Vol 1). JL Gittleman, editor. Cornell University Press, Ithaca.

Morris, R. and D. Morris (1966) The Giant Panda. Penguin Books, New York

Schaller, G. (1985) The Giant Pandas of Wolong. University of Chicago Press, Chicago.

Schaller, G., T. Quitao, K. Johnson, W. Xiaoming, S. Heming and H. Jinchu (1989) The feeding ecology of Giant Pandas and Asiatic Black Bears in the Tangjiahe Reserve, China. In Carnivore Behavior, Ecology and Evolution (Vol 1). JL Gittleman, editor. Cornell University Press, Ithaca